1. Technical Field
The invention relates to digital wireless communications systems in which delays of individual multi-path components of a time-varying fading channel are to be estimated. The digital wireless communications systems may include, for example, systems using Code Division Multiple Access (CDMA) RAKE receivers. The invention relates in particular to improving robustness of detection of new multi-path components in a radio propagation channel, as well as of tracking known paths, by reliably indicating a proper position for a path searcher window.
2. History of Related Art
In wireless communications, a physical channel between a transmitter and a receiver is formed via a radio link. In most cases, no antenna of the transmitter is narrowly focused towards the receiver. In addition to a possible direct path, many other propagation paths often exist between the transmitter and the receiver. The other propagation paths typically result from reflections from objects near the transmitter or the receiver. Rays with similar propagation distances combine at the receiver, depending on an instantaneous phase relationship, and form a distinct multi-path component. The effect of a combination of the rays depends on the instantaneous phase relationship of a carrier wavelength and also on distance differences among the rays. In the case of destructive interference, the combination of the rays leads to a significant decrease in path-gain magnitude (i.e., fading).
Performance of a CDMA receiver is improved if signal energy carried by many multi-path components is utilized. A desired improvement in CDMA receiver performance may be achieved via a RAKE receiver. In the RAKE receiver, each of a plurality of multi-path components is assigned a despreader (i.e., RAKE finger). Each of the plurality of despreaders is assigned a reference copy of a spreading code. Each of the spreading-code reference copies is delayed in time by an amount equal to a path delay of a corresponding multi-path component. Outputs of the respective despreaders are then coherently combined via a RAKE combiner to produce a symbol estimate.
The RAKE receiver preferably uses knowledge of the multi-path delays and channel-impulse values for all detected paths. To achieve a best possible signal-to-noise ratio at an output of the RAKE combiner, signal energy from as many physical paths as possible should be collected. In addition, tracking as many different physical paths as possible (i.e., maximal utilized diversity) significantly improves signal-reception robustness, since the probability of a simultaneous deep fade of all paths is reduced. Simultaneous deep fade of all paths is a phenomenon that typically leads to serious block-error-rate (BLER) degradation.
A propagation channel structure (i.e., absolute and relative delays of the individual multi-path components) does not typically remain constant over time. Due to relative movement of the transmitter, the receiver, and nearby objects, delays of existing paths may change, old paths may disappear, and new paths may appear. In addition, a frequency offset between respective circuits of the transmitter and the receiver gives rise to a clock drift. The clock drift generally manifests itself as a gradual time-axis movement of the entire delay profile. To ensure proper operation of the RAKE receiver, the changing delays of all known multi-path components should be tracked and new paths should be discovered quickly after the new paths appear.
Due to the physical channel structure, in most cases relative positions of the nearby objects change. Thus, path lengths of the new paths usually do not differ significantly from path lengths of the existing paths. The macro-structure of the channel (e.g., mountains or groups of buildings that cause signal reflections) changes relatively rarely. Therefore, most often, the delays of the new paths are relatively similar to those of the existing, known, paths. Therefore, the delays of the new paths may be detected by searching in a delay domain near the known delays of the existing paths.
FIG. 1 is a block diagram of a typical RAKE receiver. A RAKE receiver 100 includes a delay estimator block 102, a channel estimator block 104, and a RAKE despreader/combiner block 106. Received data are fed to the delay estimator block 102. The delay estimator block 102 evaluates an impulse response of a channel over a range of possible delays of the channel. A resulting delay profile, which may be a complex delay profile or a power delay profile, may then be subjected to peak detection and detected peak locations reported to the RAKE despreader/combiner block 106 as delay estimates for the multi-path components. The delay estimates are also used by the channel estimator block 104 to estimate corresponding complex channel coefficients by despreading a pilot sequence and possibly filtering results over time to reduce the effects of noise and interference. Channel parameters are estimated in collaboration between the delay estimator block 102, which determines temporal alignment of a despreader portion of the RAKE despreader/combiner block 106, and the channel estimator block 104, which estimates the complex coefficients to be used by a combiner portion of the RAKE despreader/combiner block 106. A noise-plus-interference power estimate is also made.
A simple approach to delay estimation involves evaluating an impulse response of a channel over an entire range of possible delays (i.e., maximal assumed delay spread) of the channel. A resulting complex delay profile or power delay profile may then be subjected to peak detection and detected peak locations reported by the delay estimator block 102 to the channel estimator block 104 and the RAKE despreader/combiner block 106 as delay estimates. However, processing and power-consumption expenses of frequent execution of a full path-searching routine are usually prohibitive. Therefore, typical implementations use path searchers with observation windows shorter than the full search area (i.e., a maximal assumed delay spread). In addition, for any practical delay estimation, a path search is periodically undertaken to re-scan the delay range with the purpose of detecting new paths.
A delay-estimation algorithm employed by the delay estimator block 102 extracts the path positions and finds the power delays with sufficient accuracy once the path positions have been discovered by the path searcher. A path-searcher window is positioned so that new paths are included within the path-searcher window. Since it is known with sufficient probability that the new paths will appear in the vicinity, in terms of the paths' respective delays, of the currently-known paths, the path-searcher window is usually placed so as to cover the currently-known paths.
An estimate g(τi) of a current power delay profile for delays τi(iε[1, M]) typically includes a set of recently-detected or currently-tracked paths, in which case the delays τi are usually not contiguous. g(τi) may also represent a contiguous region (τi=τ0+iΔτ) over which the path search is conducted. Other ways of representing the power delay profile are also possible.
A suitable start position I for a path searcher window of length Nw needs to be determined. A typically-used method for determining a suitable path-searcher window start position for a next path-searcher activation is based on computing a center of gravity (i.e., mean excess delay) of the presently-known power-delay-profile estimate. A center-of-gravity position estimate C is computed as follows:
                    C        =                                            ∑              i                        ⁢                                          τ                i                            ⁢                              g                ⁡                                  (                                      τ                    i                                    )                                                                                        ∑              i                        ⁢                          g              ⁡                              (                                  τ                  i                                )                                                                        (        1        )            Given C, the path-searcher window is placed so that most of the channel power is covered by the window. Because of space loss, a typical shape of the power delay profile exhibits exponential decay, such that the energy is concentrated towards the beginning of the region of interest. For reasonable coverage, the window can be placed, for example, ⅓ ahead of and ⅔ behind the value of C
      (                  i        .        e        .            ,              I        =                  C          -                                    N              w                        3                                )    .
In the case of a compact true power delay profile and a high receiver signal-to-noise ratio, C gives a consistent and reliable estimate of the true energy concentration in the channel. However, when the energy in the channel is distributed over a wide delay spread and when the signal-to-noise ratio of the power delay profile is poor, C is not so reliable. The noise-induced component of g(τi) causes a bias term that shifts the result of C towards an average non-power-weighted delay of all entries of the power delay profile. The size of the bias term depends on how far from each other the true center of gravity and the average delay are separated and also depends on the signal-to-noise ratio. In many practical cases, the bias term is large enough to shift the path-searcher window away from significant portions of the true power delay profile.
To counteract noise-induced bias effects, g(τi) may be thresholded, which removes a portion of the noise-only samples and reduces bias. However, efficient noise removal assumes the use of a rather high threshold, which may also remove channel components from, and thus distort, the power delay profile.
The noise effect may also be reduced by noise subtraction, where average noise power σg2 in the power delay profile is estimated. Instead of g(τi), g(τi)−σg2 is used in the center-of-gravity computation. In a typical implementation, the center-of-gravity computation is based on a coarse power-delay-profile estimate over a range of Nw delay values. The positions of the Np largest peaks of that power delay profile are taken as the delay values τi. The noise floor σg2 is estimated by averaging the Nn=Nw−Np smallest power-delay-profile values. However, this approach severely underestimates σg2. Since the noise floor is not removed completely, significant residual bias effect remains. To improve the robustness and precision of the center-of-gravity computation and of the resulting path-searcher window placement, an approach is needed that more adequately removes the noise floor from center-of-gravity computations.